Conventional liquid-crystal-display (LCD) backlights for large displays have conventionally employed multiple lamps to provide sufficient brightness over a large area. Typically, these spatially non-uniform, directly illuminated backlights are used for television and large display applications and contain linear arrays of fluorescent lamps with reflectors. In order to provide a uniform intensity profile from the surface of the backlight before passing through the LCD panel, volumetric diffuser plates or films are used to “spread-out” or diffuse the light from the linear array of fluorescent lights so as to eliminate the visibility of linear “hot spots” or non-uniformities in the backlight luminance.
With the emergence of light-emitting diodes (LED's) as new light sources for backlights, the LED's are often employed in linear or grid arrays and may include separate red, green and blue LED packages or all three within the same package. Backlights employing LED's not only have to improve the luminance uniformity using films, plates, and housings, but they also need to improve the color uniformity. This is often accomplished by simply increasing the amount of diffusion and not taking into account the loss in efficiency resulting by using diffusers with symmetric forward scattering profiles.
For many display applications, such as for some televisions, the viewing angle in the vertical direction is reduced such that the brightness in the forward direction is increased. This light is typically directed from higher vertical angles closer to the normal to the display using collimating films, such as prismatic brightness enhancement films. Additional films are often used such as reflective polarizers, such as dual brightness enhancement film (DBEF) from 3M (St. Paul, Minn., US). The patent literature refers to scattering reflective polarizers (U.S. Pat. Nos. 5,825,543 and 5,751,388); however, the systems described in these patents are not optimized to take into account the spatial non-uniformity of the light sources and the requirements needed to achieve spatial luminance uniformity, spatial color uniformity, as well increased head-on luminance in compact, thin, efficient systems. Using multiple films to attempt to achieve properties, such as spatial luminance uniformity, is optically inefficient due to the multiple interfacial reflections; and the manufacturing and assembly costs are higher. A more-efficient optical system for reducing the non-uniformities is needed to reduce the number of lamps (to provide a lowercost system) or to reduce the brightness of the lamps (wherein longer-lifetime or lower-cost lamps could be used) in a system with a reduced cost.
It is known that anisotropic diffusers can improve the luminance uniformity of backlights with spatially non-uniform light sources; however, greater system efficiency is desired in order to improve the system luminance and color uniformity as well as luminance uniformity while also achieving increased luminance.
Additionally, the light from backlight components such as light guides is often directed at an angle far from the normal angle. It is commonly preferred to redirect light from backlight components toward the backlight or display normal, which is the preferred viewing angle. Numerous optical films such as beaded diffusers, volumetric diffusers, collimation films, reverse prism films attempt to direct light toward the display normal with varying efficiencies and trade-offs, particularly the need for multiple film components to achieve the redirection and spread of light spatially to provide uniformity. Existing films and components are either inefficient, expensive to produce, or un-optimized with regards to providing sufficient uniformity and light re-direction for large displays.
The use of multiple films within an LCD backlight increases the production costs, increases the likelihood of dust and blemishes, and increases the part count and thickness. There is a need for fewer components for backlights that have light redirecting or collimation properties, sufficient diffusion for light homogenization and light recycling properties combined together in order to help alleviate these problems and that can be manufactured in a low-cost simple method. There is a need for a component for a backlight with increased spatial luminance uniformity and increased luminance in a particular direction. In newer LED-based backlights, increased color uniformity is important.
The use of prismatic surface structures wherein the features are greater than 10 times the wavelength of light of interest (typically greater than about 5 μm for visible light devices) on optical films can often lead to problems of Moire interference with the pixel pitch of a spatial light modulator such as an liquid crystal display in one or more directions. Also, a large range of input angles to the films can introduce spurious reflections that cause light to deviate significantly from the intended direction.